(i) Field of the Invention
The invention relates to a process and a device for the electrolytic generation of arsine (AsH.sub.3).
(ii) Description of Related Art
Gaseous hydrides play a key role in the semiconductor industry. Examples, therefore, are silane used as a precursor for the manufacture of silicon substrates or for the production of silica deposits, or even arsine used as a source of arsenic for the doping of semiconductors or for the growth of epitaxial layers of GaAsP.
The use of arsine does pose safety problems associated with the highly toxic nature of this gas, so that it has to be handled with extreme care (use of hoods) during the production, storage or even transportation thereof in the form of bottles containing a generally reduced concentration of arsine in a carrier gas.
It therefore appeared to be advantageous to perfect a method for the production (or generation) of arsine in situ (or on site) for producing arsine in situ at the inlet of the reactor using this hydride under good safety conditions and with high purity.
The electrolytic reduction of solutions containing arsenic salts rapidly appeared to be an effective solution to this problem.
The document U.S. Pat. No. 1,375,819 therefore proposes a process for the production of arsine by the electrolysis of a solution of an arsenic oxide (such as As.sub.2 O.sub.3) in an acid medium (sulphuric acid) in which potassium sulphate (K.sub.2 SO.sub.4) is also present. The electrolyser used is of the tank type, the cathode is made of carbon coated with mercury and the anode is made simply of carbon. The arrangement used results in the production of a gas which is is fact a mixture of oxygen, hydrogen and arsine. Although no precise composition is given for the mixture, it can be deduced in a simple manner from this arrangement that it does not separate the gases emitted at the cathode and at the anode, and that it does not prevent the AsO.sub.2.sup.- ions present in solution from being oxidised at the anode, thereby reducing the arsine yield accordingly.
In this context, the document U.S. Pat. No. 4,178,224 (V. R. Porter) proposes an electrolytic system for the production of arsine base on the following principle. The electrolytic cell is again of the tank type, but is made up of two concentric compartments playing the role of electrodes. These two electrodes are separated in their upper part by a solid cylindrical barrier (which is also concentric around the anode), the aim of which is to separate the gases produced at the anode and the cathode before they are discharged via the upper part of the cell. This "upper" barrier is complemented by a "lower" barrier (also cylindrical and concentric around the anode) which may or may not be continuous with the preceding barrier, the aim of which is likewise to separate the gases produced at the bubble stage, but also to allow for the passage of the H.sup.+ ions from the anode towards the cathode where they supply the arsine formation reaction. It is envisaged that this second barrier will be made of a material such as porous polypropylene or PVC, but in the latter case, a small window is provided in the lower part of the cell to allow for the passage of the H.sup.+ ions. According to this document, these two barriers could be connected together to form one single solid barrier, but, once again, an opening must then be provided in the lower part to allow for the passage of the H.sup.+ ions. The cathode is supplied with an acid solution (H.sub.2 SO.sub.4) of NaAsO.sub.2 injected between the anode and the cathode from a container exterior to the cell with the aid of a pump. Nevertheless, the results obtained show that the mixture produced at the cathode (Example 1) reaches only 20% of arsine in hydrogen in the steady state and not more than 38% at the maximum.
The document EP-A-393 897 can also be cited, once again proposing the electrolytic production of arsine. The electrolytic cell is of the tank type, containing an aqueous NaOH solution, the electrodes both consisting of arsenic. Although the arsenic yield given is high (approximately 97% in hydrogen), the throughput obtained, on the other hand, is very low (approximately 15 cm.sup.3 /h at atmospheric pressure).